Nitride semiconductors such as GaN, AlN and InN, as well as mixed crystals thereof have wide band gap and are used in devices such as high-power electronic devices or short-wavelength emitting devices. For example, the nitride semiconductor GaN has a band gap of 3.4 eV, which is larger than the band gap of Si (1.1 eV) or the band gap of GaAs (1.4 eV).
Examples of such high-power electronic devices are field effect transistors (FETs), in particular, high electron mobility transistors (HEMTs). The HEMTs including nitride semiconductors are used in, for example, high-power high-efficiency amplifiers and high-power switching devices. Specifically, in HEMTs having an AlGaN electron supply layer and a GaN electron transit layer, phenomena such as piezopolarization occur in AlGaN due to the strain ascribed to the difference in lattice constant between AlGaN and GaN, resulting in the generation of a dense two-dimensional electron gas (2DEG). By virtue of this, operations at high voltage are feasible and the HEMTs may be used in high-efficiency switching elements and high-voltage power devices in, for example, electric vehicles.
In a semiconductor device with a structure in which a gate electrode is disposed on a nitride semiconductor such as an electron supply layer, an electric field is concentrated at an end portion of the gate electrode on the drain electrode side and this field concentration may result in problems such as the breakage of the semiconductor device.
Thus, semiconductor devices are desired which have a structure that reduces the concentration of an electric field at an end portion of a gate electrode on the drain electrode side and thereby reduces the occurrence of problems such as the breakage of the semiconductor device.
The following are reference documents:
[Document 1] Japanese Laid-open Patent Publication No. 2002-359256,
[Document 2] Japanese Laid-open Patent Publication No. 2012-256923, and
[Document 3] Japanese Laid-open Patent Publication No. 2013-77629.